Scintillation bit error rate-reduction for free space optical communications systems

The free space optical (FSO) communication system has attracted many researchers from different countries, owning to its advantages such as high security, high speed and anti-interference. Among all kinds of the channels of the FSO communication system, the atmosphere channel is very difficult to deal with for two typical disadvantages at least. The one is the scintillation of the optical carrier intensity caused by the atmosphere turbulence and the other is the multipath effect by the optical scattering. A lot of studies have shown that the MIMO (Multiple Input Multiple Output) technology can overcome the scintillation of the optical carrier through the atmosphere effectively. So the background of this paper is a MIMO system which includes multiple optical transmitting antennas and multiple optical receiving antennas. A number of particles such as hazes, water droplets and aerosols exit in the atmosphere widely. When optical carrier meets these particles, the scattering phenomenon is inevitable, which leads to the multipath effect. As a result, a optical pulse transmitted by the optical transmitter becomes wider, to some extent, when it gets to the optical receiver due to the multipath effect. If the information transmission rate is quite low, there is less relationship between the multipath effect and the bit error rate (BER) of the communication system. Once the information transmission rate increases to a high level, the multipath effect will produce the problem called inter symbol inference (ISI) seriously and the bit error rate will increase severely. In order to take the advantage of the FSO communication system, the inter symbol inference problem must be solved. So it is necessary to use the channel equalization technology. This paper aims at deciding a equalizer and designing suitable equalization algorithm for a MIMO free space optical communication system to overcome the serious problem of bit error rate. The reliability and the efficiency of communication are two important indexes. For a MIMO communication system, there are two typical equalization methods. The first method, every receiving antenna has an independent equalizer without the information derived from the other receiving antennas. The second, the information derived from all of the receiving antennas mixes with each other, according to some definite rules, which is called space-time equalization. The former is discussed in this paper. The equalization algorithm concludes training mode and non training mode. The training mode needs training codes transmitted by the transmitter during the whole communication process and this mode reduces the communication efficiency more or less. In order to improve the communication efficiency, the blind equalization algorithm, a non training mode, is used to solve the parameter of the equalizer. In this paper, firstly, the atmosphere channel is described focusing on the scintillation and multipath effect of the optical carrier. Then, the structure of a equalizer of MIMO free space optical communication system is introduced. In the next part of this paper, the principle of the blind equalization algorithm is introduced. In addition, the simulation results are showed. In the end of this paper, the conclusions and the future work are discussed.

Relay-assisted cooperative free space optical (FSO) communication system is a powerful technique, which offers advantages like fading mitigation and significant gain in spatial diversity due to shorter transmission hops. Even so, it can be hampered by jamming activities which in turn degrade the overall system performance. In this paper, the effect of relay jamming in the decode-and-forward (DF) protocol based cooperative FSO communication system, is introduced and studied thoroughly. The probability density function (pdf) of a random variable containing mixture of the negative exponential and Gaussian random variables is derived. The error performance of the considered FSO system is analysed over this new kind of additive mixture noise for different probabilities of jamming. By utilizing the derived pdf, a closed-form expression of the average bit error rate (BER) of the DF FSO relaying is obtained. A comparison of the BER performance of maximum-likelihood (ML) detector and a derived threshold based detector is also provided. It is observed that the derived threshold based detector offers almost similar performance as that of the ML detector. A thorough numerical study is also provided by considering different pointing error parameters of jammer's and user's channels.

To realize laser high-speed data transmission of multiple nodes in space information networks, a one-to-two space laser communication system was designed and carried out for outfield experiments. Based on the experimental results, the causes affecting the transmission BER of free-space optical (FSO) communication system are comprehensively analyzed from both the platform vibration characteristics and the atmospheric turbulence characteristics. According to the vibration characteristics of the airship platform, the effect of coarse and fine composite axis tracking technology on the BER of FSO communication system was studied. Based on the log-normal probability density function (PDF) of atmospheric turbulent light intensity scintillation, the relationship between the structure constant of atmospheric refractive index and the system BER was derive to quantitatively analyze the effect of atmospheric turbulence on space laser communication. The experimental results show that a communication BER of 9.45E-9 is achieved at a communication distance of 2km and a communication rate of 2.5Gbps in a weak turbulent channel. The research results play a positive role in promoting the development of FSO communication technology under atmospheric turbulent channels.

We present a novel, self-consistent analytical model of Gaussian-beam propagation through the atmospheric turbulence by solving the paraxial wave equation in a fractional-dimension space of dimension D, in the range 2 < D ≤ 3, corresponding to the effective spatial dimension experienced by the beam under given turbulent conditions in a free space optical (FSO) communication system. The well-known refractive index structure parameter (C n2) has been mapped from D = 2.668 (C n2≈10-13, strong fluctuations) to D = 2.999 (C n2≈10-16, weak fluctuations) in our simple analytical model, whereas D = 3 corresponds to the ideal case of free-space propagation under zero turbulence. Finally, an optimization problem is developed to mitigate the effects of atmospheric turbulence, leading to efficient transceiver design for the FSO communication system to ensure the reliability of links under varying atmospheric turbulence.

The beam motion is one of the main causes for major power loss which severely degrades the performance of the Free Space Optical Communication (FSOC) system. Designing a suitable controller to correct the beam motion to increase the beam stability at a point becomes significant. This paper presents an investigation on the performance of two types of controller designed for aiming a laser beam to be at a particular spot under dynamic disturbances generated at the transmitter. The experimental nonlinear input---output data mapping is used as the principal components for the controller design. The first design is based on the Taguchi method while the second is the artificial neural network--cognitive method. These controllers process the beam pointing (beam location) information from an opto-electronic position detector and then generate the necessary outputs to steer the beam with fast steering mirror. Conceptually, the controller design problem is addressed with looking the position error information to decide the level of nonlinear static output map for generating the correction control information. The pipelined-parallel architecture of both controllers are developed in a field programmable gate array (FPGA) and installed at the receiver station. Evidence of the suitability and the effectiveness of the proposed controller in terms of prediction exactness, response to impulse, scintillation, Q-factor and bit error rate are provided through experimental results obtained from the 155 Mbps FSOC data link set-up established for the horizontal range of 0.5 km at an altitude of 15.25 m.

In a free space optical communication (FSOC) system, atmospheric turbulence will increase the bit error ratio (BER) and impair FSOC link reliability. Since computational temporal ghost imaging (CTGI) has anti-interference, we present an FSOC system over atmospheric turbulence based on CTGI. The simulation results show that the BER performance of CTGI is better than on-off keying under different atmospheric turbulence regimes. To improve the performance of the CTGI scheme, the influence of the number of transmission samples and code length is analyzed. It is shown that BER performance improves with the increment of the number of samples, while code length has no impact. This scheme provides an idea for reliable communication over atmospheric turbulence and an important reference for improving wireless optical communication in an extreme environment.

A MPR optimization algorithm for FSO communication system with star topology

Clouds are key driver in the performance of free space optical communication (FSOC) systems. Clouds are composed of liquid water and/or ice crystals and depending on the physical thickness can produce atmospheric fades easily exceeding 10 dB. In these more common cases, impacts on FSOC systems may be severe. On the other hand, there are times when cloud fades may be as low as 1 or 2 dB as a result of thin, ice crystal based cirrus clouds. In these cases, the impacts on FSOC communication collectors may be limited. The ability to characterize the distribution and frequency of clouds are critical in order to understand and predict atmospheric impacts. A cloud detection system has been developed and applied to produce high resolution climatologies in order to investigate these impacts. The cloud detection system uses geostationary, multi-spectral satellite imagery at horizontal resolutions up to one kilometer and temporal resolutions up to fifteen minutes. Multispectral imagery from the visible wavelengths through the longwave infrared is used to produce individual cloud tests which are combined to produce a composite cloud analysis. The result represents a high spatial and temporal resolution climatology that can be used to derive accurate Cloud Free Line of Sight (CFLOS) statistics in order to quantify atmospheric effects on optical communication systems. The Lasercom Network Optimization Tool (LNOT) is used along with a mission CONOPS and the cloud database to find configuration of geographically diverse ground sites which provide a high availability system.

In this paper, we propose and demonstrate a free space optical (FSO) communication system with 10 Gbit s−1 on-off keying traffic, which is integrated in a passive optical network (PON) for broadcasting wireless data. After a transmission distance of 25 km of single-mode fiber and a free space transmission length of 6 m, the observed optical sensitivity of the FSO signal can reach −35.2 dBm within the forward error correction level at the bit error ratio of 3.8 × 10−3. Here, according to the obtained results of experiment and simulation, the FSO signal can support 2048 remote optical wireless units (OWUs) for wireless access via the PON network in free space transmission lengths of 0 to 160 m. Moreover, the proposed FSO-PON system also can support 1024, 512 and 256 remote OWUs in free space transmission lengths of 250, 350 and 500 m, respectively.

Free space optical (FSO) communication system is a groundbreaking technology in the field of communication systems. Non-orthogonal multiple access (NOMA), on the other hand, currently being used in fifth-generation (5G) wireless systems, is a powerful technique for establishing communication for multiple users using the same time and frequency resources. In this paper, the performances of NOMA and orthogonal multiple access (OMA) in an FSO-based communication system are studied. In the FSO communication system, two base station units are considered to be connected to a central unit for FSO backhauling using an uplink-fixed NOMA scheme. Bit error rate (BER), ergodic capacity, and energy efficiency (EE) performances of the NOMA-based FSO system are studied and compared with an OMA-based FSO system. It is found that the NOMA-based system provides approximately 10% of the ergodic capacity gain and increases EE by 37%-60% for a given BER compared to the OMA-based system.

Performance of free space optical communication systems over exponentiated Weibull atmospheric turbulence channel for PPM and its derivatives

In free-space optical communication (FSOC) systems, atmospheric turbulence can bring about power fluctuations in receiver ends, restricting channel capacity. Relay techniques can divide a long FSOC link into several short links to mitigate the fading events caused by atmospheric turbulence. This paper proposes a Reinforcement Learning-based Relay Selection (RLRS) method based on Deep Q-Network (DQN) in a FSOC system with multiple transceivers, whose aim is to enhance the average channel capacity of the system. Malaga turbulence is studied in this paper. The presence of handover loss is also considered. The relay nodes serve in decode-and-forward (DF). Simulation results demonstrate that the RLRS algorithm outperforms the conventional greedy algorithm, which implies that the RLRS algorithm may be utilized in practical FSOC systems.

Free Space Optical Communication, FSO, has come into prominence recently as compared to the other optical communication systems. FSO has impressive advantages such as high data rate, wide bandwidth, better security, and robustness to electromagnetic interference, EMI. The performance of the system depends on the factors such as link distance and meteorological conditions. In this study, a reference FSO system which utilizes Bessel, Butterworth, Chebyshev and Gaussian optical filters is implemented in OptiSystem 16.1 simulation software. Effect of meteorological conditions such as clear, hazy and rainy weather are simulated for various link distances, namely 800, 1000 and 1500 meters. The performances of these optical filters according to the above mentioned factors are evaluated by measurement metrics such as BER, Q factor and eye height.

Free space Optical (FSO) communication systems are severely laid down with the weather situations. Atmospheric turbulence is one of the essential facts that degrade the overall performance of a FSO system even under smooth sky circumstance. Coding is one of the techniques that may be applied to improve the overall performance of a communication system. Moreover, it's miles very crucial to apply proper coding approach to enhance the performance. This paper affords an analytical method to evaluate the overall performance of LDPC and STBC coded FSO communication system under the impact of strong atmospheric turbulence. Evaluation is done to find the expressions for the output photodetector current and signal to noise power ratio. The average bit error rate is evaluated numerically thinking about gamma-gamma distributions for the atmospheric turbulence. Analytical results show that for similar type of system configuration in FSO, STBC provides better performances than LDPC code.

LDPC-coded OAM shift-keying FSO communication system with dual-pattern CNN demodulator